This application claims the priority of Korean Patent Application No. 2002-31607, filed Jun. 5, 2002, which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to an apparatus and a method, which can output continuous-tone images as binary-coded data, and more particularly, to an apparatus and method for binary-coding images so as to improve the quality of output images.
2. Description of the Related Art
A continuous-tone image consists of pixel values in a two-dimensional space. A binary coding apparatus limits output values to binary-coded values. For example, the binary-coding apparatus can be used for an apparatus, such as a facsimile, a printer, a digital copy machine, and a liquid crystal display (LCD) panel.
When a continuous-tone image is input to the binary-coding apparatus, the apparatus simplifies continuous-tone values to two levels, i.e., 0 or 1, and outputs the simplified values. To this end, a conventional binary-coding apparatus masks input pixels to critical values of the locations of the pixels that are presently processed.
FIG. 1 is a block diagram illustrating a conventional binary-coding apparatus. Referring to FIG. 1, a conventional binary-coding apparatus includes a counter unit 100, a memory address generator 110, a memory 120, and a comparator 130.
The counter unit 100 outputs location information on an input pixel I(x,y) in a two dimensional image. The counter unit 100 outputs the location information while considering that continuous-tone images are input pixel-by-pixel from the top left corner to the bottom right corner of the two dimensional image. Accordingly, the counter unit 100 includes an X-axis location counter 101 and a Y-axis location counter 102. The X-axis location counter 101 outputs an X-axis value of the input pixel I(x,y) in the two dimensional image. The Y-axis location counter 102 outputs a Y-axis value of the input pixel I(x,y) in the two dimensional image.
The memory address generator 110 generates a one dimensional memory address corresponding to the X-axis and Y-axis location information output from the counter unit 100 and a control signal that controls the read mode of the memory 120.
The memory 120 stores mask critical values corresponding to each pixel of the two dimensional image. When the memory address and the control signal are transferred from the memory address generator 110, the memory outputs a predetermined mask critical value M(x,y) corresponding to the input pixel I(x,y).
The comparator 130 compares the value of the input pixel I(x,y) with the mask critical value M(x,y) to output a binary pixel value B(x,y) of the input pixel I(x,y). For example, if the value of the input pixel I(x,y) is larger than the mask critical value M(x,y), the comparator 130 outputs 1 as the binary pixel value B(x,y). If the value of the input pixel I(x,y) is not larger than the mask critical value M(x,y), the comparator 130 outputs 0 as the binary pixel value B(x,y).
Therefore, the quality of the binary-coded image output from the binary-coding apparatus is determined according to the resolution, the distribution and the size of a mask of the mask critical values stored in the memory 120. In other words, if the elements of the mask critical values are regularly arranged in the memory 120, regular patterns are formed in the image output from the binary-coding apparatus.
Bayer Dither's mask critical value matrices, which have been widely used, are shown in FIGS. 2A through 2C. FIG. 2A illustrates a 4×4 matrix of pixels and FIG. 2B illustrates an 8×8 matrix of pixels. FIG. 2C is the mask critical value matrix for rotate Bayer Dither of the 4×4 of pixels. When the size of a mask critical value matrix, i.e., a mask size, is small, arbitrary patterns may occur in a binary-coded image due to irregularities in the critical values located at visual edges and the edge values of a peripheral critical value arrangement.
A stochastic mask having irregularly arranged critical values, high frequency elements, and a size larger than a conventional mask can be used. Since the stochastic mask can represent a large number of critical values, the tone of an image output from a binary-coding apparatus can be improved.
The above-described masks are iteratively used for a two dimensional image as shown in FIG. 3. Here, FIG. 3 is a conceptual diagram illustrating a method of outputting a binary-coded image using the masking method of FIG. 1, wherein masks having the same size are iteratively used.
However, experimentally, a binary-coded image with reduced regular patterns due to the iterative use of the mask can be outputted only when the mask is larger than 64×64 pixels. As the size of a mask increases, it is possible to reduce the amount of patterning in an output binary-coded image. However, in that case, a memory capacity for storing mask critical values also increases. In addition, since the mask includes a high frequency element, such as blue noise, the high frequency element of an input image signal is reduced by the mask critical values. Therefore, the image quality of boundary elements, which are visually important elements in an image, is lowered as shown in FIG. 4. Here, the mask critical values are dispersed in the blue noise. In other words, the blue noise occurs because the critical values having similar values are separate from each other. A mask in which critical values are dispersed produces an excellent quality image; however, the mask cannot produce boundary portions due to the dispersed critical values. In other words, if the mask is used, the tone of a background having a large number of low frequency elements is represented well; however, the tone of boundary portions of characters or fine images is damaged. In addition, the technology to output the binary-coding image proposed has been previously disclosed in U.S. Pat. No. 5,825,940.